When's the last time you purchased a CD? For an ever-growing number of consumers, the last time they pulled the cellophane off of a CD jewel case is but a distant memory. Many high school-aged kids have never had the experience, and likely never will.  Instead, music consumption has moved online to channels both legal and not. Peer-to-peer file sharing, MP3 stores and cloud music lockers have been joined by all-you-can-stream subscription apps. These services are quickly becoming a huge hit with mainstream consumers, and may even help reduce piracy. But which one offers the best experience?  In terms of pricing, there's essentially no difference between the key players. Rdio, MOG and Spotify all offer limited free accounts, which they ultimately hope to convert to premium, ad-free, mobile-accessible accounts for $5 or $10 per month. That leaves these services to duke it out over their feature sets, and it's a close fight.  When you stack all of these services up against one another, Rdio and Spotify both come out on top, for slightly different reasons.  Rdio and Spotify are, for the most part, remarkably similar. They both have huge libraries of music from all four major labels and many independent labels. Both offer apps for iPhone, iPad, Android, Blackberry and Windows Phone, and each has a presence on miscellaneous other digital entertainment devices. They both integrate with Facebook, if "frictionless sharing" is your thing.  They also make very good competition for one another. Each service has a feature or two that bests the other provider in some way. Spotify: An Infinitely Extensible Library of Music  Spotify's biggest advantage is its ability to merge your local music collection with its massive cloud-based library. It would be impossible for any streaming service to offer access to every song in existence, but Spotify does the best job of letting the user plug the gaps. Syncing one's music through Spotify can eliminate the need to use the platform's native music player and even free iOS users from the shackles of iTunes. Another major perk of Spotify is its third-party add-on apps. Late last year, the company opened up its new platform so developers can build HTML5-based apps that run within the Spotify desktop client. While the apps have yet to be ported over to any mobile platform, the functionality they offer is extremely promising.  Rdio: Superior User Experience  While Spotify has certainly captured the bulk of the available buzz since its U.S. launch last year, it doesn't entirely outdo Rdio.  What the two-year-old startup lacks in library extensibility, it makes up for with exceptionally designed apps on mobile and the desktop. Unlike Spotify, Rdio also has a Web app, allowing users to enjoy the service in the browser without having to download software.  Mysteriously absent from Spotify's apps is a way to easily navigate one's collection by artist or album. Instead, browsing of content is based on playlists and search. It's functional, but inconvenient, not even bothering to borrow from a convention that has existed since the first iPod. Rdio doesn't make this mistake: The music is arranged by artist and then broken down into albums, as it should be.  Rdio also appears to have certain popular albums that Spotify lacks. Pink Floyd's studio albums come to mind. Overall, Spotify has a bigger selection of music, so this is another thing that more or less balances out between the two, depending on one's tastes.